How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Early symptoms of phoma basal rot on romaine lettuce consist of yellowing and wilting of lower leaves. Affected plants may later show an uneven appearance because one side of the foliage grew normally while the diseased side of the plant remained stunted and short. As plants mature, the entire plant becomes stunted and eventually wilts and collapses. The wilting and collapsing symptoms are somewhat similar to those caused by Sclerotinia minor (causal agent of lettuce drop) and Botrytis cinerea (causal agent of gray mold of lettuce).

Examination of affected plants reveals distinct, black cavities (sunken areas) on the crown and upper taproot. These cavities extended far into the crowns and roots, resulting in extensive weakening of the plant; these plants could be broken off easily at ground level. The black cavities are characteristically firm and lack any sign of fungal growth, sclerotia, or spores. This disease is primarily found on romaine; other lettuce types are susceptible but typically are infected at low incidences.

COMMENTS ON THE DISEASE

Phoma exigua is a soilborne fungus. Phoma basal rot was first documented in California in 2000 and can cause significant losses in romaine plantings. The disease has been found in coastal counties, but has not yet been confirmed in Huron or desert regions. Little is known about the epidemiology of this disease. A similar disease on greenhouse grown lettuce was reported in 1965 in the United Kingdom.

MANAGEMENT

Avoid planting romaine cultivars in fields having histories of this problem. Protectant fungicides applied after thinning the crop can reduce Phoma basal rot problems. Treat before plants become too large and direct applications to the base of the young plants.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental quality.
A.	AZOXYSTROBIN
	(Quadris)	12.3–15.4 fl oz	4	0
	MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
	COMMENTS: Alternate with a fungicide, such as sulfur, that has a different mode of action Group number. See label for special phytotoxicity precautions.
B.	BOSCALID
	(Endura)	8–12 oz	12	14
	MODE OF ACTION GROUP NAME (NUMBER1): Carboxamide (7)
	COMMENTS: Do not make more than 2 applications/season.
	+  Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. # Acceptable for use on organically grown produce. 1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions (for more information, see http://www.frac.info/). Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode of action Group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

PUBLICATION

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey County
R. M. Davis, Plant Pathology, UC Davis

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